The importance of the calcium ion in metabolic processes has only recently been recognized. In many cases specific interactions occur between the calcium ion and a variety of proteins and enzymes, such as occur in the muscle contractile system. The function of the calcium ion in these systems is obscure and the ability to use the calcium ion itself to investigate the interaction is limited. Since we have previously shown that many lanthanide ions can substitute for the calcium ion in several enzymes and still maintain enzymatic activity, we plan to substitute lanthanide ions into systems requiring calcium and use the spectral and magnetic properties of these ions to probe the binding of the metal ion. Application of techniques such as fluorescence, difference absorption spectroscopy (both isotropic shifts and relaxation) will be made to single model systems (e.g., amino acids and peptides), the protein thermolysin and several proteins in the muscle contractile system. The results of the analysis will be applied to the interpretation of the mode of binding and function of these metal ions in the protein system. Since the lanthanides appear to be good general replacements for the calcium ion protein systems, extrapolation of these results should be capable of being extended directly to the important calcium containing proteins.